Code:
Option Explicit

Public LoginSucceeded As Boolean

Private Sub cmdCancel_Click()
    'set the global var to false
    'to denote a failed login
    LoginSucceeded = False
    Me.Hide
End Sub

Private Sub cmdOK_Click()
With Adodc1.Recordset
            .MoveFirst
                Do Until .EOF = True
                    If txtUserName.Text = .Fields("UserName").Value
...